The reduction of noise in video signals, either by reducing the spatial bandwidth, or by averaging stationary objects over several different temporal samples, is well known. These processes have been found not only to enhance the subjective appearance of pictures but also to facilitate the data compression of video by removing less-visually-significant information prior to the compression process.
A commonly-used technique is to combine the video signal from a point in a current frame with the signal from a corresponding point in a previous frame or frames. This is usually implemented as a recursive (IIR) filter, which may be motion compensated, where the recursion coefficient is varied in response to a motion dependant control signal. Stationary areas may be averaged over several frames, whereas moving areas may not be averaged at all.
A known implementation of this technique is to subtract two consecutive frames to obtain a signal containing noise and motion; this signal is passed through a non-linear function which attenuates small signals but allows large signals to pass. The output of the non-linear processing is then added to the earlier frame to obtain a noise-reduced output. In this arrangement the instantaneous gain of the non-linear process effectively controls the degree of recursion. When the instantaneous gain is zero the previous frame is output; and when the gain is unity the added previous-frame information is cancelled by the subtracted previous-frame information, and the processing has no effect. Usually the shape of the non-linearity, and hence the gain characteristic, is varied in response to a motion adaptation signal so that minimal processing is applied in moving areas.
In spite of this adaptation, the performance may not be adequate in moving areas and a further adaptation to a non-recursive, spatial bandwidth reduction noise reducer may be used. Typically this spatial processing “cores” the high-frequency content of the signal by attenuating low amplitude components (which are mainly noise), whilst allowing larger high frequency components to pass.
Where video images have been derived from film, the grain of the film may be a subjective impairment and these known video noise reduction techniques may be used to make it less objectionable. However, film grain is a contribution to the well-recognised and appreciated “film-look” and removing it by these prior art methods sometimes leads to subjectively unacceptable results.